Carbon fibers produced in gas phase growth are known to have much more excellent mechanical and electrical properties due to excellent crystal orientation than those of conventional carbon fibers, and their commercial production has been desired.
Heretofore, the carbon fibers according to the gas phase growth have been produced by placing in an electric furnace a substrate of alumina or graphite, on a surface of which is distributed an ultra-fine metal particle catalyst of iron, nickel etc., which in turn is reduced by a hydrogen gas, and thereafter decomposing thermally a mixed gas of a hydrocarbon gas (such as benzene) and a carrier gas (such as hydrogen) on the catalyst. However, such procedure has several disadvantages as follows:
(1) high irregularity of fiber length on account of temperature differences with resulting uneven fiber length and ununiform dispersion of catalyst with resulting coarseness and closeness of the fiber formation, (2) since an organic compound gas as a carbon source is consumed by reaction, there occurs a difference in a concentration of the organic compound between an inlet and an outlet to the reactor, so that sizes of the resulting fibers vary in the vicinity of the inlet or the outlet, (3) the carbon fiber is formed only on the surface of the substrate but the central zone of the reactor is not involved in the reaction, resulting in a poor yield, and (4) independent and separate processes, such as dispersion of the ultrafine particles onto the substrate, placement of said substrate within a furnace, reduction of oxidized metal particles with hydrogen at the elevated temperature, formation of carbon fibers on the substrate through thermal decomposition of the organic compound, decrease of the furnace temperature, and removal of the carbon fibers are required, so that continuous production is difficult and thus the productivity is low.
In order to solve the above problems and to improve productivity and quality of the carbon fibers, the inventors have earnestly studied continuous processes for producing carbon fibers in the gas phase growth to develop an improved process in which a mixed gas of a carbon compound gas, an organic transition metal compound gas and a carrier gas is heated at a temperature ranging from 600.degree. C. to 1300.degree. C., and has filed a patent application therefor (Japanese Patent Application No. 162606/83).
Subsequently, the inventor has further continued the studies to find out the inorganic transition metal compounds may be efficiently employed as well, which have relatively high volatility or which are soluble in an organic solvent.
Accordingly, an object of the invention is to provide a process for preparing carbon fibers in gas phase growth, which can solve the problems in the prior art and improve the productivity.